This blog is devoted to the discussion of all aspects of synthetic organic chemistry and related sciences. Curly Arrow is run by a synthetic organic chemist based in Copenhagen, Denmark. Contributions from readers are always welcome and should be emailed to curlyarrow@gmail.com

Tuesday, November 28, 2006

Let's talk about TLCs Part 2 - Hanessian's Stain

Don't you just love that feeling when you are checking your reaction mixture by TLC and no matter which stain you use nothing appears on the plate. If your compound also isn't UV active you've got a real problem. Well the good news is that there is one stain that will do the job for you - Hanessian's Stain! Hanessian's stain is an excellent multi-purpose stain that when used the right way usually gives blue spots (TLC plate A). Since it is a water based stain it requires vigorous heating for development. However, if you overdo it the entire plate goes dark blue (TLC plate B).

Dissolve ammonium molybdate and cerium sulfate in water (with heating if required) followed by careful addition of concentrated sulfuric acid. Sometimes an insoluble residue is observed. If that happens remove it by filtration. Cerium sulfate can be replaced with Cerium ammonium sulfate which is significantly cheaper (Recipe 2). Hanessian's stain is used just as previously described for the Vanillin Stain although it requires more vigorous heating. Because rather harsh heating is required this stain may prove inefficient with volatile compounds. Also keep in mind that this a very sensitive stain so even trace impurities can appear as significant spots on your TLC plate. D!

Oooops a bit of an inconsistency between formulas and names in the recipe. Should all be OK now. There are numerous variations on this recipe. It doesn't seem to make any difference if you use Cerium Ammonium Nitrate / Sulfate / etc. As far as I know it all works fine so it's basically down to availablility and price. I've corrected the previous recipe and added another version of the stain. I don't know if the stain is named after Steve Hanessian. Maybe we should drop him an email. He might also be able to give us the original recipe. BTW how sweet is it to have a stain names after you? All the other dudes have reactions and reagents named after them but Hanessian is the only person to have named a stain (as far as I know). Oh and yes it is also sometimes called CAM (cerium ammonium molybdate) stain. D!

This is the best general stain there is. I have run into compounds that don't work with phosphomolybdic acid and CAM has always saved my bacon!(mmmm...baaaacon...gggrrrggggg gggrrrrgggg) This is my first choice stain.

The thing I like about anisaldehyde is that often times analogs in a series will stain with very similar color. In a past project it made quick work of finding my product (stained a characteristic blue color)amongst a myriad of other stuff.

it certainly is nice when products have a characteristic color on TLC. I make compounds that very commonly turn red after sitting on the TLC plate in air for an hour or so (they have sulfur in them). This has often been more indicative than the crude NMR data.

I would have to disagree regarding vanilin stains capabilities when compared to Hanessian stain. Vanilin is my preferred stain (see post at Curly Arrow by following the TLC label) because it is easy to handle and it colours different types of compounds differently. However, Hannessian stain is significantly more sensitive. I have had many compounds that didn't show up with vanilin at all but are easily detected with Hanessian stain. D!

At our lab we have been using PAM for a few years now, but lately when trying to apply quantitative measuring procedures problems came up; so i ended up here today, thinking about moving to CAM. My question is about the heating - what do you use for heating? what temperature and for how long do you heat it? I would greatly appreciate your help :)

Ok, so it probably takes a few minutes only for the spots to appear. I forgot to ask - as was mentioned PMA is quite light-sensitive reagent - is it the same with CAM, or it is more stable? And thanks for the quick answer :)

Morridin, spots appear within a minute with a powerful heatgun. Spots do fade realtively fast but different spots fade at different rates. I usually photocopy my plates the same day that I have developed them. D!